The large volume production of chlorine and caustic (sodium hydroxide) needed to meet the demands of a modern society has led to the development and nearly exclusive use of electrolysis of aqueous solutions of sodium chloride to produce these essential materials.
Electrolytic cells of three general types are in general use. Initially, the so-called mercury cell was used in which a brine electrolyte was electrolyzed in a cell utilizing a liquid mercury cathode and an anode spaced from the surface thereof to produce chlorine gas and sodium-mercury amalgam. The product amalgam was then treated to remove the sodium as sodium hydroxide.
More recently, diaphragm cells have been developed, this type of cell now providing the majority of the production in chlorine and caustic.
A diaphragm-type electrolytic cell is comprised of a pair of electrode compartments which are separated by a diaphragm, usually made of asbestos or modified asbestos, one compartment containing an anode, the other a cathode. In applying the cell to use, brine (aqueous sodium chloride solution) is fed continuously into the anode compartment. Hydraulic pressure causes the brine to flow through the diaphragm to the cathode compartment. A flow rate of brine is maintained in excess of the conversion rate so that back migration of hydroxide ions is minimized. Chlorine gas is produced at the anode while hydrogen gas is evolved at the cathode, sodium ions combining with the hydroxyl group remaining after the electrolysis of water to form sodium hydroxide solution. Thus, the catholyte is a solution of sodium hydroxide and unconverted sodium chloride and other impurities which must be further processed to "pure" concentrated sodium hydroxide solution. Residual sodium chloride solution is returned to the cell for further processing.
Dimensionally stable anodes and various coating compositions therefor have permitted greater cell efficiencies since the anode-cathode gap may be narrowed significantly.
The use of the dimensionally stable anode with a substantially hydraulically impermeable ion-exchange membrane as an anode-cathode separator has the potential for even greater cell efficiency and substantially reduced production costs as compared with the use of a diaphragm separator.
Membrane cells permit only certain ions to migrate between the anolyte and catholyte. This results in a substantial improvement in the purity of the caustic catholyte since most metallic impurities and chlorine are retained in the anolyte. The post-electrolysis purification cost is thus substantially reduced. Furthermore, membrane cells produce a caustic of higher concentration than diaphragm cells thereby reducing or eliminating the cost of post-electrolysis concentration.
In order to increase the production and efficiency of electrolytic cells, filter press type structures have been proposed for the use of a plurality of cells connected in series or parallel to produce chlorine, alkali metal hydroxides and hydrogen.
In a bipolar filter press type structure, a plurality of cell units are connected in series in a filter press in which each electrode except those located at each end of the series acts as an anode on one side and a cathode on the other side. The space between adjacent bipolar electrodes is divided by a separator such as a diaphragm, modified diaphragm or membrane into anode and cathode compartments. Typically, an alkali metal halide solution is fed into the anode compartment where halogen gas is generated at the anode. Alkali metal ions migrate through the separator to the cathode compartment, there to form alkali metal hydroxide while hydrogen gas is liberated at the cathode. The product alkali metal hydroxide in the catholyte is then processed, as needed, to the desired purity.
A bipolar electrode is an electrode without direct metallic connection with a source of electric current, one face of which acts as an anode and the opposite face of which acts as a cathode when electric current is passed through the cell.
While the filter press-type electrolytic cell structure allows some economics of operation, the entire cell structure must be disassembled to remove and replace any faulty components of the structure. During this time, the entire cell is out of operation for the period of time required for maintenance and repair. The loss of operating time thus reduces the economy of operation gained by using such a structure.
The patent of Cottam, et al., U.S. Pat. No. 3,242,059 is illustrative of a filter press-type cell bank in which a plurality of anode-cathode pairs are located within a common enclosure, the electrodes being connected in series to form bipolar electrodes. The connection is effected by corrugated titanium sheets which also act to separate the electrolytes of adjacent cells. If any one component of this type of cell bank requires replacement, it is necessary to shut down the entire bank since it is an integral structure.
Various types of enclosed single-cell units connected in series have been proposed to alleviate the problem of complete shutdown and disassembly of a cell bank. However, the cell units are generally interconnected in series by a plurality of heavy external busbars whereby an anode of one cell is connected to the cathode of an adjacent cell. A connector of this type is described in Emery, et al., U.S. Pat. No. 3,565,783 issued Feb. 23, 1971. With the use of this type of external connector, there is still a considerable amount of production time lost in removing and reattaching the fasteners connecting the busbars to the cell units.
Another type of unitary cell is described in U.S. Pat. No. 4,017,375, issued Apr. 12, 1977, in which a plurality of cell units are welded together to provide a bipolar filter press cell bank. This structure incorporates conductive strips between two adjacent cell units which strips are welded to both cell units in order to establish electrical connection and provide cooling air space therebetween. The entire filter press structure is then encased in concrete to seal against corrosion and to provide a solid structure for absorbing the clamping stresses of a filter press type structure. As with other filter press structures, it is necessary to disassemble the entire cell bank in order to replace any one defective or worn component.